Arsenic cell stabilization valve for gallium arsenide in-situ compounding

ABSTRACT

A quartz arsenic cell having a stabilizing valve used to generate hot arsenic vapor which is flowed into liquid gallium, to provide a melt of liquid gallium arsenide from which a crystal can be pulled. The stabilizing valve prevents negative relative pressure from occurring in the quartz arsenic cell, and thus prevents the molten material from being sucked back up into the quartz arsenic cell.

BACKGROUND AND SUMMARY OF THE INVENTION

The present invention relates to preparation of monocrystalline compoundsemiconductors, particularly gallium arsenide.

To successfully grow large single crystals of GaAs by the liquidencapsulated Czochralski (LEC) technique, the GaAs starting materialmust first be compounded from elemental gallium and arsenic. The purestand most cost effective way to do this is to compound at low pressure inthe crystal puller just prior to growth. See Pekarek, "Apparatus forSynthesis of Gallium Arsenide Under Liquid Encapsulant", Czech. J. Phys.B20 (1970), page 857, which is hereby incorporated by reference. The waythis has been done at Texas Instruments is shown in FIG. 1. Aradiatively heated quartz cell is used to force arsenic vapor through aninjection tube into the liquid gallium melt, where it reacts chemicallyto form GaAs. The approach shown in the figure is very similar to thatused by others in the industry, most notably Hewlett-Packard. SeePuttbach et al, "Liquid Encapsulated Synthesis and Czochralski Growth ofGaAs at Low Pressure", Fifth International Conference on Vapor Growthand Epitaxy (Coronado 1981), Abstracts p. 75. The disadvantage to thisapproach is that the pressure inside of the cell tends to drop belowthat outside of the cell at various times during the compounding cycle,and this can cause the liquid Ga and GaAs to be sucked up into the cell,ruining the run and creating a safety hazard. This can be avoided byusing very high flux of arsenic, e.g. by externally heating the arseniccell. However, a large flux of arsenic has two major disadvantages:first, the resulting high condensation of arsenic in the furnace chambermeans that growth cannot occur in the same chamber where compounding hasbeen done. This means that a transfer step is necessary, with largepossibilities for contamination of the semiconductor material. Secondly,a very rapid introduction of the arsenic leads to poor control of meltcomposition, which leads to seriously degraded control over thecrystal-growth process. Control is degraded since an unknown amount ofescaped arsenic vapor will enter the chamber atmosphere and condense onthe colder parts of the furnace chamber walls. Control of meltcomposition is important, since the melt must be about 2% arsenic-rich(or have a slightly larger excess of arsenic), of a stoichiometriccrystal is to be pulled: if the melt is very much richer in arsenic,e.g. 5% excess arsenic or more, the crystal which is sought to be pulledwill twin, or cannot be seeded, or will be pulled as polycrystallinematerial. If the melt has less than 2% atomic of arsenic, the crystalspulled will tend to be nonstoichiometric, and have a high concentrationof arsenic vacancies. Thus, while the use of very high arsenic flux ratesolves the suction problem, it also severely degrades control of theprocess generally.

Thus it is an object of the present invention to provide a method forcompounding gallium arsenide in-situ immediately prior to crystalgrowth, in which good control of the melt composition is retained.

It is a further object of the present invention to provide a method forcompounding gallium arsenide in-situ immediately prior to crystalgrowth, without introducing a large amount of arsenic into the chamberatmosphere.

It is a further object of the present invention to provide a method forcompounding and growing semiconductor-grade gallium arsenide crystals,without requiring the compounded gallium arsenide to be exposed to theatmosphere prior to crystal growth.

The present invention has alleviated the suction problem by adding astabilization valve to the cell as shown in FIG. 2. This valve preventsthe pressure inside the cell from ever becoming less than that outside.

Thus, the primary object of the present invention is to permit in-situcompounding of gallium arsenide, as a preliminary to pulling a crystalof gallium arsenide, without the risk of introducing molten gallium orgallium arsenide into the arsenic chamber.

Introduction of the hot melt into the arsenic chamber is dangerousbecause the melt is many hundreds of degrees above the sublimationtemperature of arsenic. Thus, essentially all the arsenic in the quartzcell will immediately vaporize. This rapid thermal vaporization is verylikely to break the quartz arsenic cell, and dump the melt which hasflowed up into the cell back down into the remaining melt in thecrucible. The result of this is that gallium and gallium arsenide willbe splashed all over the chamber, causing significant damage to theapparatus and necessitating a lengthy cleanup. Moreover, the suddenvaporization of much of the arsenic charge may even bring the internalpressure in the puller high enough that some arsenic vapor escapes intothe atmosphere. Such arsenic vapor will immediately react to formarsenic trioxide dust, which is extremely poisonous.

According to the present invention there is provided: a system forcompounding gallium arsenide, comprising: crucible means for holding amelt of gallium and gallium arsenide; an arsenic sublimation cell, saidsublimation cell comprising an arsenic vapor injection tube downwardlyextending from an upper portion of said arsenic cell; means forpositioning said arsenic cell so that said arsenic vapor injection tubeextends into the interior of said crucible; wherein said arsenic cellfurther comprises a stabilizing valve, said stabilizing valve comprisinga ball check valve to prevent low relative pressure inside said arseniccell.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be described with reference to theaccompanying drawings wherein:

FIG. 1 shows schematically an apparatus, for in-situ compounding ofgallium arsenide at a low pressure immediately prior to Czochralskigrowth, in which the improvement of the present invention can beincorporated;

FIG. 2 shows an arsenic cell, as used in the apparatus of FIG. 1, whichincorporates a stabilizing valve according to the present invention; and

FIG. 3 shows a second type of in-situ compounding apparatus, using anexternally-heated rather than radiatively-heated arsenic cell, in whichthe present invention can be incorporated.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows schematically the key elements of an apparatus for in-situlow pressure compounding of gallium arsenide prior to liquidencapsulated Czochralski growth of a monocrystalline gallium arsenideboule. The whole apparatus shown in normally encased in a furnacechamber, and a heating element (such as a resistance heater (preferred)or an induction coil) is provided around the melt. Initially, thearsenic coil is charged with lumps of solid arsenic, and the crucible isfilled with metallic gallium. Of course, both starting elements must beof extremely high purity, to achieve the desired eventual purity. Anencapsulant, such as boric oxide, is also loaded into the crucible. Thisencapsulant is required for the liquid-encapsulated Czochralski growth.

To begin the compounding process, the crucible is heated to melt theinitial charge of gallium and boric oxide. This occurs at a lowtemperature, namely 29° C. for the gallium and 450° C. for the boricoxide. In the present case, where the arsenic is added relativelyslowly, the heat generated by the exothermic reaction between galliumand arsenic is a minor perturbation. The exothermic nature of thereaction is of importance only if the reactants are initially put inintimate physical contact prior to reaction. As the reaction proceedsmore heat must be supplied to the arsenic cell in order to heat theupper side (away from the direct radiation) above the sublimation pointof arsenic (about 620° C.). In the presently preferred embodiment, thisis achieved by lowering the cell closer to the melt. Usually the powerinput to the heater will not be changed during the compounding. Thepreferred procedure is to heat the gallium/boric oxide mixture to themelting point of gallium arsenide (about 1250° C.), bring the quartzcell into such a position such that the injection tube is in contactwith the gallium, and then the cell/crucible combination is graduallylowered in the heater until all the arsenic has evaporated and reacted.In the presently preferred embodiment of the present invention, theinitial charge of gallium is 708 g, the initial charge of arsenic is 777g, and the arsenic is sublimed for compounding at a rate between zeroand 10000 preferably about 1500) grams per hour over the wholecompounding cycle. However, the rate of sublimation can be allowed tochange during the compounding cycle, if desired. Eventually, much largerruns would be desirable, to pull multi-kilogram crystals.

After compounding is essentially complete, the quartz arsenic cell islifted from the melt and retracted, and the crystal pulling process canbegin. The steps in the actual crystal pulling process are well known inthe art of material science generally. Since the melting temperature ofa gallium arsenide melt is somewhat variable with the exact compositionof the melt, the melt is initially brought to a temperature which isslightly above the melting point of gallium arsenide, which is in theneighborhood of 1250° C. The seed lift mechanism is used to lower theseed until it comes in contact with the melt surface, and thetemperature is gradually lowered until growth begins. The onset ofcrystal growth is easily determined visually by looking at the meniscusformed where the seed tip contacts the melt. That is, the melt surface(which is highly reflective) will be dimpled upward slightly to makecontact with the edges of the seed. When the diameter of the meniscusincreases, this indicates that crystal growth has begun, and controlledwithdrawal of the seed, together with separate respective rotations ofthe seed and of the melt, are now used to pull a crystal from most ofthe mass of the melt. Preferably a slight nitrogen overpressure (e.g. 23or more psi of nitrogen) is maintained in the chamber during compoundingand growth, but any other non-reactive gas can be used.

The present invention teaches an arsenic cell as shown in FIG. 2. Thiscell is also made of quartz, as in the prior art, but includes astabilizing valve. This stabilizing valve is preferably made entirely ofquartz, including the ball check and stem. Such quartz check valves areseparately available off-the-shelf from commercial suppliers.

When this compounding cell is used, the materials processing proceeds aswith a conventional apparatus, except that the chance of negativerelative pressure in the arsenic cell, and of material being sucked upinto the arsenic cell, is now completely avoided. Thus, the apparatus issafer and more reliable, and control of the compounding rate is muchsimpler. In the presently preferred embodiment, where the arsenic cellis sized to contain an original charge of 777 g of arsenic (in lumpswhich are roughly 1/4" across), the stabilizing valve has an orifice0.20" in diameter, with a ball check which is 0.375" in diameter.However, these dimensions are not at all critical. The entrance from thestabilizing valve to the arsenic compounding cell should, of course,have holes no larger than the minimum size of the granular arsenic use.However, this is not critical, since granular arsenic which falls intothe body of the stabilizing valve will typically sublime first, sincethe stabilizing valve sees more radiant heating from the melt than therest of the quartz arsenic cell does. The chosen size (weight) of theball check itself is merely dependent on how far up the injection tubeit is permissible to permit the melt to climb. The inside diameter ofthe arsenic injection tube is not particularly relevant, but, in thepresently preferred embodiment, is about 21 mm.

Thus, this stabilizing valve has a very simple construction, suitablefor high-temperature operation without introduction of impurities, andavoids a major safety hazard previously associated with gallium arsenidein-situ compounding. The present invention is not only applicable togallium arsenide, but is also used for other compound semiconductormaterials where in-situ compounding is desired to be used.

The present invention is not solely applicable to compounding of galliumarsenide, but is also applicable to compounding of other compoundsemiconductors, where one of the elements is volatile. Thus, the presentinvention is of particular interest for compounding of phosphides,particularly gallium phosphide and indium phosphide. In-situ synthesisand growth of indium phosphide has recently been described in Farges, "AMethod for the `In-Situ` Synthesis and Growth of Indium Phosphide in aCzochralski Puller", 59 Journal of Crystal Growth 665 (1982), which ishereby incorporated by reference. That is, wherever compounding occursby vaporization of a more volatile element in an enclosed cell andinjection of that volatile element through an injection tube into a meltfor compounding, the present invention teaches applying a check valve tothe closed vaporization cell to prevent the occurrence of a localnegative pressure condition in the vaporization cell. Other volatileelements from which it may be desired to compound semiconductors ofinterest include tellurium, sellenium, and sulfur. However, none of theII-VI compounds are nearly as interesting as gallium arsenide or indiumphosphide. It is also, of course, not strictly necessary that thepresent invention be applied only to semiconductor materials, althoughit is in semiconductor materials that the desiderata of high purity,very accurate stoichiometry, and very good crystal quality are ofprimary importance.

In addition, the stabilization valve of the present invention is notonly applicable to a compounding system as shown in FIG. 1, but can alsobe applied in other systems, such as that shown in FIG. 3. In the systemof FIG. 3, the injection tube is slanted to offset the sublimation cell,so that the sublimation cell does not block the path of the seed forcrystal pulling. Thus, in such an arrangement, the sublimation cellrequires only a small degree of vertical travel, to withdraw theinjection tube for the melt, or, if adequate temperature controls areavailable, the sublimation cell can be fixed in place. The advantage ofsuch a system is that it is inherently better adapted for pulling largecyrstals, since nearly all of the width of the lower part of the pullerchamber can be used for the crucible and liner. In all such systems, thecrucible will typically be made of a strong refractory material, such asgraphite, and a liner of a very inert material, such as quartz, will beinserted in the crucible.

In the presently preferred embodiment, a modification of puller such asa Hamco CG800 is used. This widely known puller has an upper chamber(the "pull chamber") which is separated from the lower chamber (the"furnace chamber") by a valve. Initially, the crucible (which is in thefurnace chamber) is charged with Ga and B₂ O₃ as discussed above, and onAs cell according to the present invention is attached to the seed liftmechanism in place of the seed chuck. The As cell used in thisembodiment is attached to a resistance heater. After both chambers areclosed, evacuated, and backfilled with nitrogen, the crucible is heated,the valve is opened, the As cell is lowered until the injection tube tipis submerged in the melt, and the resistance heater on the As cell isturned on until the As is all sublimed. (This can be determined eitherby monitoring the weight of the cell, or by visually observing the meltsurface.) The As cell is then retracted, and the valve between upper andlower chambers is closed. The As cell is then removed, and the seedcheck (with a seed) is remounted. After the upper chamber is evacuatedand backfilled, the valve is opened, the seed is lowered, and growthproceeds as described above.

The preferred way to fill the As cell uses a removable center insert.The cell is held upside down and charged with As chunks, and then aperforated insert is emplaced and crimped in the arsenic vapor injectiontube. However, many other expedients will serve to prevent As chunksfrom falling through the injection tube into the melt.

It will be apparent to those skilled in the art that the presentinvention may be practiced in a wide range of modifications andvariations, and the invention is accordingly not limited except asspecified in the accompanying claims.

What is claimed is:
 1. A system for compounding gallium arsenide,comprising:crucible means for holding a melt of gallium and galliumarsenide; an arsenic sublimation cell, said sublimation cell comprisingan arsenic vapor injection tube downwardly extending from an upperportion of said arsenic cell; means for positioning said arsenic cell,so that said arsenic vapor injection tube extends into the interior ofsaid crucible; wherein said arsenic cell further comprises a stabilizingvalve, said stabilizing valve comprising a ball check valve to preventlow relative pressure inside said arsenic cell.
 2. The system of claim1,further comprising seed withdrawl means for lowering a seed intocontact with said melt and for withdrawing the seed at a controlledrate.
 3. The system of claim 1, wherein said positioning means providesretractable positioning.